Onnowpurbo: Created page with "Principles of Redistribution The capabilities of the IP routing protocols vary widely. The protocol characteristics that have the most bearing on redistribution are the differ..."

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Created page with "Principles of Redistribution The capabilities of the IP routing protocols vary widely. The protocol characteristics that have the most bearing on redistribution are the differ..."

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Principles of Redistribution
The capabilities of the IP routing protocols vary widely. The protocol
characteristics that have the most bearing on redistribution are the
differences in metrics and administrative distances, and each protocol's
classful or classless capabilities. Failure to give careful consideration to
these differences when redistributing can lead to, at best, a failure to
exchange some or all routes or, at worst, routing loops and black holes.
Metrics
The routers of Figure 11-1 are redistributing static routes into OSPF,
which then advertises the routes to other OSPF-speaking routers. Static
routes have no metric associated with them, but every OSPF route must
have a cost. Another example of conflicting metrics is the redistribution of
RIP routes into EIGRP. RIP's metric is hop count, whereas EIGRP uses
bandwidth and delay. In both cases, the protocol into which routes are
redistributed must be able to associate its own metrics with those routes.
So the router performing the redistribution must assign a metric to the
redistributed routes. Figure 11-2 shows an example. Here EIGRP is
being redistributed into OSPF, and OSPF is being redistributed into
EIGRP. OSPF does not understand the composite metric of EIGRP, and
EIGRP does not understand OSPF cost. As a result, part of the
redistribution process is that the router must assign a cost to each EIGRP
route before passing it to OSPF. Likewise, the router must assign a
bandwidth, delay, reliability, load, and MTU to each OSPF route before
passing it to EIGRP. If incorrect metrics are assigned, redistribution will
fail.
Figure 11-2. When routes are redistributed, a metric that is
understandable to the receiving protocol must be assigned
to the routes.Case studies later in this chapter show how to configure routers to assign
metrics for purposes of redistribution.
Administrative Distances
The diversity of metrics presents another problem: If a router is running
more than one routing protocol and learns a route to the same
destination from each of the protocols, which route should be selected?
Each protocol uses its own metric scheme to define the best route.
Comparing routes with different metrics, such as cost and hop count, is
like comparing apples and oranges.
The answer to the problem is administrative distances. Just as metrics
are assigned to routes so that the most preferred route may be
determined, administrative distances are assigned to route sources so
that the most preferred source may be determined. Think of an
administrative distance as a measure of believability. The lower the
administrative distance, the more believable the protocol.
For example, suppose a router running RIP and EIGRP learns of a route
to network 192.168.5.0 from a RIP-speaking neighbor and another route
to the same destination from an EIGRP-speaking neighbor. Because ofto the same destination from an EIGRP-speaking neighbor. Because of
EIGRP's composite metric, that protocol is more likely to have
determined the optimum route. Therefore, EIGRP should be believed
over RIP.
Table 11-1 shows the default Cisco administrative distances. EIGRP has
an administrative distance of 90, whereas RIP has an administrative
distance of 120. Therefore, EIGRP is deemed more trustworthy than RIP.
Table 11-1. Cisco default administrative distances.
Route Source Administrative Distance
Connected interface [*] 0
Static route 1
EIGRP summary route 5
External BGP 20
EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
External EIGRP 170
Internal BGP 200
Unknown 255Unknown
255
[*]
Recall from Chapter 3, "Static Routing," that when a static route refers to an interface
instead of a next-hop address, the destination is considered to be a directly connected
network.
Although administrative distances help resolve the confusion of diverse
metrics, they can cause problems for redistribution. For example, in
Figure 11-3, both Gehrig and Ruth are redistributing RIP-learned routes
into IGRP. Gehrig learns about network 192.168.1.0 via RIP and
advertises the network into the IGRP domain. As a result, Ruth learns
about network 192.168.1.0 not only from Combs via RIP, but also from
Meusel via IGRP.
Figure 11-3. Network 192.168.1.0 is being advertised to
Ruth via both RIP and IGRP.Example 11-1 shows Ruth's route table. Notice that the route to
192.168.1.0 is an IGRP route. Ruth has chosen the IGRP route because,
compared to RIP, IGRP has a lower administrative distance. Ruth will
send all packets to 192.168.1.0 over the "scenic route" through Meusel,
instead of sending them directly to Combs.
Example 11-1. Although the optimal route to network
192.168.1.0 from Ruth is through Combs, out S0, Ruth is
instead routing to that network through Meusel, out S1.
Ruth#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route
Gateway of last resort is not set
I
192.168.1.0/24 [100/16100] via 192.168.5.1, 00:00:00, Seri
I
192.168.2.0/24 [100/12576] via 192.168.5.1, 00:00:00, Seri
I
192.168.3.0/24 [100/12476] via 192.168.5.1, 00:00:01, Seri
I
192.168.4.0/24 [100/10476] via 192.168.5.1, 00:00:01, Seri
C
192.168.5.0/24 is directly connected, Serial1
C
192.168.6.0/24 is directly connected, Serial0
Ruth#
Split horizon prevents a routing loop in the network of Figure 11-3. Both
Gehrig and Ruth initially advertise subnet 192.168.1.0 into the IGRP
domain, and the four IGRP routers eventually converge on a single path
to that subnet. However, the convergence is unpredictable. This condition
can be seen by rebooting routers Lazzeri and Meusel. After the reboot,
Ruth's route table shows that it is using Combs as the next-hop router toreach 192.168.1.0 (Example 11-2).
Example 11-2. The convergence of the network in Figure
11-3 is unpredictable. After a reboot of the routers, Ruth is
now routing to 192.168.1.0 through Combs.
Ruth#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route
Gateway of last resort is not set
R
192.168.1.0/24 [120/1] via 192.168.6.2, 00:00:23, Serial0
I
192.168.2.0/24 [100/12576] via 192.168.5.1, 00:00:22, Seri
I
192.168.3.0/24 [100/12476] via 192.168.5.1, 00:00:22, Seri
I
192.168.4.0/24 [100/10476] via 192.168.5.1, 00:00:22, Seri
C
192.168.5.0/24 is directly connected, Serial1
C
192.168.6.0/24 is directly connected, Serial0
Ruth#
Convergence after the reboot is not only unpredictable but also slow.
Example 11-3 shows Gehrig's route table approximately three minutes
after the reboot. It is using Lazzeri as a next-hop router to subnet
192.168.1.0, but pings to a working address on that link fail. Lazzeri's
route table (Example 11-4) shows the problem: Lazzeri is using Gehrig
as a next-hop router. A routing loop exists.
Example 11-3. Soon after the reboot, Gehrig is routing
packets to 192.168.1.0 via Lazzeri.
Gehrig#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route, o - ODR
Gateway of last resort is not set
I
192.168.1.0/24 [100/16100] via 192.168.3.2, 00:02:38, Seri
C
192.168.2.0/24 is directly connected, Ethernet0
C
192.168.3.0/24 is directly connected, Serial0
I
192.168.4.0/24 [100/10476] via 192.168.3.2, 00:00:29, Seri
I
192.168.5.0/24 [100/12476] via 192.168.3.2, 00:00:29, Seri
I
192.168.6.0/24 [100/14476] via 192.168.3.2, 00:00:39, Seri
Gehrig#ping 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 sec
.....
Success rate is 0 percent (0/5)
Gehrig#
Example 11-4. Lazzeri is routing packets to 192.168.1.0 via
Gehrig, creating a routing loop. Notice the age of the route.
Lazzeri#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route
Gateway of last resort is not set
I
192.168.1.0/24 [100/12100] via 192.168.3.1, 00:04:21, Seri
I
192.168.2.0/24 [100/8576] via 192.168.3.1, 00:00:33, SeriaC
192.168.4.0/24 is directly connected, Serial1
I
192.168.5.0/24 [100/10476] via 192.168.4.2, 00:00:53, Seri
I
192.168.6.0/24 [100/12100] via 192.168.3.1, 00:02:32, Seri
Lazzeri#
Here's the sequence of events leading to the loop:
1. While Lazzeri and Meusel are rebooting, both Gehrig and Ruth have
route table entries showing network 192.168.1.0 as reachable via
Combs.
2. As Lazzeri and Meusel become active, both Gehrig and Ruth send
IGRP updates that include subnet 192.168.1.0. Simply by the "luck
of the draw," Ruth sends its update slightly earlier than Gehrig does.
3. Meusel, receiving Ruth's update, makes Ruth the next-hop router
and sends an update to Lazzeri.
4. Lazzeri, receiving Meusel's update, makes Meusel the next-hop
router.
5. Lazzeri and Gehrig send updates to each other at about the same
time. Lazzeri makes Gehrig the next-hop router to 192.168.1.0
because its route is metrically closer than Meusel's route. Gehrig
makes Lazzeri the next-hop router to 192.168.1.0 because its IGRP
advertisement has a lower administrative distance than Combs RIP
advertisement. The loop is now in effect.
Split horizon and the invalid timers will eventually sort things out. Lazzeri
is advertising 192.168.1.0 to Meusel, but Meusel continues to use the
metrically closer route via Ruth. And since Ruth is the next-hop router,
split horizon is in effect for 192.168.1.0 at Meusel's S1 interface. Meusel
is also advertising 192.168.1.0 to Lazzeri, but Lazzeri sees Gehrig as
metrically closer.
Lazzeri and Gehrig see each other as the next-hop router to 192.168.1.0,so they will not advertise the route to each other. The route will age in
both of their route tables until the invalid timer expires (Example 11-5).
Example 11-5. When the invalid timer for the route to
192.168.1.0 expires, the route is declared unreachable and
the holddown timer is started.
Lazzeri#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route
Gateway of last resort is not set
I
192.168.1.0/24 is possibly down, routing via 192.168.3.1,
I
192.168.2.0/24 [100/8576] via 192.168.3.1, 00:00:57, Seria
C
192.168.3.0/24 is directly connected, Serial0
C
192.168.4.0/24 is directly connected, Serial1
I
192.168.5.0/24 [100/10476] via 192.168.4.2, 00:01:25, Seri
I
192.168.6.0/24 is possibly down, routing via 192.168.3.1,
Lazzeri#
When Lazzeri's invalid timer expires, the route to 192.168.1.0 will be put
into holddown. Although Meusel is advertising a route to that network,
Lazzeri cannot accept it until the holddown timer expires. Example 11-6
shows that Lazzeri has finally accepted the route from Meusel, and
Example 11-7 shows that Gehrig is successfully reaching 192.168.1.0
through Lazzeri. It took more than nine minutes for these two routers to
converge, and the route they are using is still not the optimal route.
Example 11-6. After the holddown timer for 192.168.1.0
expires, Lazzeri accepts the route advertised by Meusel.Lazzeri#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route
Gateway of last resort is not set
I
192.168.1.0/24 [100/14100] via 192.168.4.2, 00:00:27, Seri
I
192.168.2.0/24 [100/8576] via 192.168.3.1, 00:00:02, Seria
C
192.168.3.0/24 is directly connected, Serial0
C
192.168.4.0/24 is directly connected, Serial1
I
192.168.5.0/24 [100/10476] via 192.168.4.2, 00:00:28, Seri
I
192.168.6.0/24 [100/12476] via 192.168.4.2, 00:00:28, Seria
Lazzeri#
Example 11-7. Gehrig can now reach subnet 192.168.1.0 via
Lazzeri.
Gehrig#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route, o - ODR
Gateway of last resort is not set
I
192.168.1.0/24 [100/16100] via 192.168.3.2, 00:00:32, Seri
C
192.168.2.0/24 is directly connected, Ethernet0
C
192.168.3.0/24 is directly connected, Serial0
I
192.168.4.0/24 [100/10476] via 192.168.3.2, 00:00:33, Seri
I
192.168.5.0/24 [100/12476] via 192.168.3.2, 00:00:33, Seri
I
192.168.6.0/24 [100/14476] via 192.168.3.2, 00:00:33, SeriType escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 sec
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/
Gehrig#
Administrative distances can cause even worse problems than the sub-
optimal routes, unpredictable behavior, and slow convergence of the
previous example. For example, Figure 11-4 shows essentially the same
network as in Figure 11-3, except that the links between the IGRP routers
are Frame Relay PVCs. By default, IP split horizon is turned off on Frame
Relay interfaces. As a result, permanent routing loops will form between
Lazzeri and Gehrig and between Meusel and Ruth. Subnet 192.168.1.0
is unreachable from the IGRP domain.
Figure 11-4. Because IP split horizon is turned off by
default on Frame Relay interfaces, permanent routing
loops will form in this network.Several tools and strategies exist to prevent routing loops when
redistributing. The administrative distances can be manipulated, and
route filters or route maps can be used. Chapter 13 covers route filters,
and Chapter 14 covers route maps. These chapters also demonstrate
techniques for changing administrative distances.
Redistributing from Classless to Classful
Protocols
Careful consideration must be given to the effects of redistributing routes
from a classless routing process domain into a classful domain. To
understand why, it is necessary to first understand how a classful routing
protocol reacts to variable subnetting. Recall from Chapter 5, "Routing
Information Protocol (RIP)," that classful routing protocols do not
advertise a mask with each route. For every route a classful router
receives, one of two situations will apply:
The router will have one or more interfaces attached to the major
network.
The router will have no interfaces attached to the major network.
In the first case, the router must use its own configured mask for that
major network to correctly determine the subnet of a packet's destination
address. In the second case, only the major network address itself can
be included in the advertisement because the router has no way of
knowing which subnet mask to use.
Figure 11-5 shows a router with four interfaces connected to subnets of
192.168.100.0. The network is variably subnettedtwo interfaces have 27-
bit masks, and two interfaces have 30-bit masks. If the router is running a
classful protocol such as IGRP, it cannot use the 27-bit mask to derive30-bit subnets, and it cannot use the 30-bit mask to derive 27-bit subnets.
So, how does the protocol cope with the conflicting masks?
Figure 11-5. If this router is running a classful routing
protocol, what mask should it choose?
In Example 11-8, debugging is used to observe the IGRP advertisements
sent by the router in Figure 11-5. Notice that subnet 192.168.100.128/27
is advertised out interface E0, which has a 27-bit mask, but neither
192.168.100.4/30 nor 192.168.100.8/30 is advertised out that interface.
Similarly, 192.168.100.8/30 is advertised out interface S1, which has a
30-bit mask, but neither 192.168.100.96/27 nor 192.168.100.128/27 is
advertised out that interface. The same situation applies to all four
interfaces. Only subnets of 192.168.100.0 whose masks match the
interface mask are advertised. As a result, IGRP-speaking neighbors on
interfaces E0 and E1 will have no knowledge of the 30-bit subnets, and
IGRP-speaking neighbors on interfaces S0 and S1 will have no
knowledge of the 27-bit subnets.
Example 11-8. A classful routing protocol will not advertise
routes between interfaces whose masks do not match.
O'Neil#debug ip igrp transactions
IGRP protocol debugging is onO'Neil#
IGRP: sending update to 255.255.255.255 via
subnet 192.168.100.128, metric=1100
IGRP: sending update to 255.255.255.255 via
subnet 192.168.100.96, metric=1100
IGRP: sending update to 255.255.255.255 via
subnet 192.168.100.4, metric=8476
IGRP: sending update to 255.255.255.255 via
subnet 192.168.100.8, metric=8476
O'Neil#
Ethernet0 (192.168.
Ethernet1 (192.168.
Serial0 (192.168.10
Serial1 (192.168.10
This behavior of only advertising routes between interfaces with matching
masks also applies when redistributing from a classless routing protocol
into a classful routing protocol. In Figure 11-6, the subnets of the OSPF
domain are variably subnetted, and Paige is redistributing OSPF-learned
routes into IGRP.
Figure 11-6. Paige is redistributing its OSPF-learned routes
into IGRP.
As Example 11-9 shows, Paige knows about all of the subnets in both the
OSPF and the IGRP domain. And because OSPF is classless, the router
knows which masks are associated with each subnet connected to
Gibson. Paige's IGRP process is using a 24-bit mask; therefore,
172.20.113.192/26 and 172.20.114.48/28 are not compatible and are not
advertised (Example 11-10). Notice that IGRP does advertise172.20.112.0/24 and 172.20.115.0/24. The result is that the only subnets
within the OSPF domain that Leonard knows of are the ones with a 24-bit
mask (Example 11-11).
Example 11-9. Paige knows about all six subnets of Figure
11-6, either from OSPF, IGRP, or a direct connection.
Paige#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route, o - ODR
Gateway of last resort is not set
172.20.0.0/16 is variably subnetted, 6 subnets, 3 masks
O
172.20.113.192/26 [110/74] via 172.20.112.1, 00:01:35,
C
172.20.112.0/24 is directly connected, Ethernet1
O
172.20.115.0/24 [110/80] via 172.20.112.1, 00:01:35, Et
I
172.20.110.0/24 [100/1600] via 172.20.111.1, 00:00:33,
C
172.20.111.0/24 is directly connected, Ethernet0
O
172.20.114.48/28 [110/74] via 172.20.112.1, 00:01:35, E
Paige#
Example 11-10. Only the OSPF-learned routes with a 24-bit
mask are successfully redistributed into the IGRP domain,
which is also using a 24-bit mask.
Paige#debug ip igrp transactions
IGRP protocol debugging is on
Paige#
IGRP: received update from 172.20.111.1 on Ethernet0subnet 172.20.110.0, metric 1600 (neighbor 501)
IGRP: sending update to 255.255.255.255 via Ethernet0 (172.20.1
subnet 172.20.112.0, metric=1100
subnet 172.20.115.0, metric=1100
Paige#
Example 11-11. The only subnets within the OSPF domain
known at Leonard are the ones with a 24-bit mask.
172.20.113.192/26 and 172.20.114.48/28 are unreachable
from here.
Leonard#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inte
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external
E1 - OSPF external type 1, E2 - OSPF external type 2, E
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c
U - per-user static route, o - ODR
Gateway of last resort is not set
172.20.0.0/24 is subnetted, 4 subnets
I
172.20.112.0 [100/1200] via 172.20.111.2, 00:00:48, Eth
I
172.20.115.0 [100/1200] via 172.20.111.2, 00:00:48, Eth
C
172.20.110.0 is directly connected, Ethernet0
C
172.20.111.0 is directly connected, Ethernet1
Leonard#
The configuration section includes case studies that demonstrate
methods for reliably redistributing from a classless routing protocol to a
classful routing protocol.

==Pranala Menarik==

* [[IPv6: Advanced Routing]]

IPv6: Route Control - Route Redistribution - Revision history

Onnowpurbo: Created page with "Principles of Redistribution The capabilities of the IP routing protocols vary widely. The protocol characteristics that have the most bearing on redistribution are the differ..."